1. Field of the Invention
The present invention relates to a wireless communication system and wireless terminal in the wireless communication system which performs transmission on the basis of carrier sense.
2. Description of the Related Art
In a wireless communication system which periodically ensures a transmission slot, a given wireless terminal executes a sequence of notifying another wireless terminal of the reception status of the given wireless terminal, the state of a wireless link estimated by reception processing, or transmission control information.
For example, in a cellular system used for cellular phones, a wireless base station periodically receives feedback information from a wireless terminal to transmit data to the wireless terminal by performing transmission control such as optimal rate selection or transmission power control.
Among wireless communication systems designed to acquire a transmission slot by carrier sense, there are also known a wireless communication system and wireless terminal, in which when a given wireless terminal receives data from another wireless terminal, the given wireless terminal generates information as an individual data frame, such as the reception status of the given wireless terminal and the designation of a transmission rate for the destination wireless terminal, which influences transmission control on the destination wireless terminal, and transmits the data frame to the destination wireless terminal.
In a wireless communication system which periodically ensures a transmission slot, a transmission slot for notifying a wireless terminal to which data is transmitted of feedback information is ensured, and hence no contention for a transmission slot occurs between the wireless terminal and another wireless terminal. This makes it unnecessary to perform transmission timing control for ensuring a stable notification. In contrast to this, in a wireless communication system which acquires a transmission slot by carrier sense, feedback information needs to be transmitted with contention with other wireless terminals.
In the sequence of transmitting feedback information in a conventional wireless communication system designed to perform carrier sense, since feedback information itself is transmitted as individual data upon carrier sense under contention with other wireless terminals, feedback information cannot be stably and efficiency notified. In addition, a receiving wireless terminal cannot timely notify the transmitting wireless terminal of feedback information in accordance with the timing of need for the feedback information in transmission by the transmitting wireless terminal.
Assume that, in particular, a wireless communication system designed to perform carrier sense simultaneously includes wireless terminals which use different modulation schemes or differ in handling of transmission antenna configurations, e.g., using a reception scheme matching transmission from a single antenna or a reception scheme matching transmission from a plurality of antennas. In this case, even if a frame containing channel reservation information is transmitted in advance so as to prevent contention for a predetermined period of time, no effect is produced on a wireless terminal incapable of decoding the frame.